The “page proxy” is that tiny document icon that you see in the title bar of an open document, just to the left of the document name. That page proxy is "live," and you can do a number of things with it, just as if it were the file icon itself.

I use the Dictionary/Thesaurus application so much that I put it in my
Dock and leave it open all the time instead of taking the extra click
to get to the Dashboard. But there’s an even faster way to find the definition or synonym of a word—the Dictionary panel.